What specific characteristic of major arms is less densely packed in the Orion Spur environment?

Answer

Vast clouds of nebulae and massive blue stars

Major arms are characterized by intense new star formation, rich in massive, hot, short-lived blue stars and vast clouds of nebulae, which is less dense in the Orion Spur.

What specific characteristic of major arms is less densely packed in the Orion Spur environment?
galaxyastronomysolar systemMilky WaySagittarius Arm